Dereferencing a pointer to variable beyond its lifetime
suggest changeint* foo(int bar) { int baz = 6; baz += bar; return &baz; /* (&baz) copied to new memory location outside of foo. */ } /* (1) The lifetime of baz and bar end here as they have automatic storage * duration (local variables), thus the returned pointer is not valid! */ int main (void) { int* p; p = foo(5); /* (2) this expression's behavior is undefined */ *p = *p - 6; /* (3) Undefined behaviour here */ return 0; }
Some compilers helpfully point this out. For example, gcc
warns with:
warning: function returns address of local variable [-Wreturn-local-addr]
and clang
warns with:
warning: address of stack memory associated with local variable 'baz' returned [-Wreturn-stack-address]
for the above code. But compilers may not be able to help in complex code.
- Returning reference to variable declared
static
is defined behaviour, as the variable is not destroyed after leaving current scope. - According to ISO/IEC 9899:2011 6.2.4 §2, “The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime.”
- Dereferencing the pointer returned by the function
foo
is undefined behaviour as the memory it references holds an indeterminate value.
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